1. Field of the Invention
The present invention relates to formals of polyhydroxy compounds and more particularly to an improved method of preparing them.
2. Description of the Prior Art
Formals of compounds having the formula CH.sub.2 OH(CHOH).sub.n CH.sub.2 OH have been prepared by reaction with formaldehyde under acid conditions and, in many cases, are well characterized. The processes of the art, however, involve reaction times in the order of several hours or days, and isolation procedures which often are complex and time-consuming. For example, the triformals of mannitol and of sorbitol were prepared by Schultz and Tollens [Ber. 27, 1892 (1894) and Ann. 289, 20 (1896)] by heating the polyol with 40% formaldehyde and concentrated hydrochloric acid under a reflux condenser for 3/4 to 11/2 hour, followed by filtration after several hours. No yields were given. Ness et al [J. Am. Chem. Soc. 65, 2215 (1943)], who obtained only a 40% yield of mannitol triformal by the Schultz and Tollens procedure, improved it by heating the mannitol, 10 g, with formalin, 17.5 g, and concentrated hydrochloric acid, 20 ml, at 50.degree. C. for 5 days. After filtering off the triformal, more heating of the filtrate for 4 days at 50.degree. C. brought the total yield to 93%. Ouchi and So [J. Macromol. Sci. Chem A 10, 1599 (1976)] substituted sulfuric acid for hydrochloric acid in preparing sorbitol triformal by the Ness et al method, but again, long reaction periods and large volumes of water were involved.
More recent efforts in the preparation of various acetals of pentaerythritol offer no clues on the possible improvement of the conventional preparations of polyol formals. Renner et al (U.S. Pat. No. 3,978,088), for instance, prepare diacetals with dimeric .beta.-OH-pivaldehyde in hydrochloric acid by a 24 hour reaction. Fruhstorfer et al (U.S. Pat. No. 3,621,034), on the other hand, boil pentaerythritol with dodecylthiopropanal for 31/2 hours, while Kray et al (U.S. Pat. No. 2,915,530) obtain their diacetals with e.g. acrolein after a 4 hour reflux period in the presence of 0.1% phosphoric acid.
Finally, Zey et al (U.S. Pat. No. 4,076,727) still more recently disclosed the preparation of cyclic acetal from polyols, e.g. trimethylolpropane, and an aldehyde such as formaldehyde and butyraldehyde, by refluxing such compounds in, e.g. benzene for a period of at least 4 hours. The possibility of higher temperatures, i.e., up to 150.degree. C., is mentioned, solvent permitting it is assumed. In any event, Zey et al appear to be committed to a conventional process to obtain the acetals that they need for the synthesis of acrylic esters.
In summary, the processes that have just been reviewed, as well as several other existing approaches, variously suffer from one or more of the following shortcomings, namely: long reaction times--hours or days; long cooling times; inferior yield of product; time-consuming procedures for isolation; use of solvents; high expenditure of energy; and inadaptability to continuous production.
In view of this situation, it is the principal object of this invention to provide a process which will be free of any and all of the art limitations just enumerated.